The invention relates to a unique rigid radially expansible drum for manufacture of large pneumatic tires such as heavy-duty tires and to the use of such drum on existing bias tire building machines to produce tubeless and tube-type radial tires.
For more than three decades automatic or semi-automatic tire building machines have been employed by tire manufacturers to build bias-ply tires. The complete green tire is built-on a cylindrical or high-crown fixed-diameter drum and thereafter expanded to toroidal shape in a vulcanizing mold. (see U.S. Pat. Nos. 2,381,379; 2,394,318; 2,464,020 and 2,488,340). The tire building operations are carried out commercially on automatic or semi-automatic tire building machines, such as the Model 49 or Model 59J described hereinafter or other tire building machines sold by National Rubber Machinery Company (NRM) or other manufacturers, including the NRM Models 39, 40, 50, 59H, 61B and 611.
The circumferential belts used on radial and bias/belted tires prevent building of such tires by the old single-stage process used for bias-ply tires because of the pantographing problem when the tire is expanded from the cylindrical to the toroidal shape. For this reason the carcasses of radial tires must be expanded before the circumferential belts and the tread are applied to form the green tire.
In a typical process for making radial tires, the tire building operation is carried out in two stages. In the first stage the carcass is formed and the underply is turned over the beads. This may involve use of an expansible drum as in U.S. Pat. Nos. 3,560,301; 3,637,450, 3,684,621; 3,740,293; 3,784,437; 4,131,500; 4,151,035 and 4,312,696. The carcass is usually formed in a generally cylindrical shape as shown, for example, in U.S. Pat. Nos. 3,536,566; 3,560,301; 3,616,059; 3,740,293 and 3,795,564. In the second stage the carcass is expanded substantially before applying the circumferential belts and the tread is disclosed, for example, in U.S. Pat. Nos. 3,560,301; 3,560,302; 3,565,062; 3,637,450; 3,674,604; 3,684,621; 3,740,293; 3,784,437; 3,833,445; 4,214,939; 4,243,451; and 4,312,696. A typical machine has bead locking means which expands against the inside of the bead rings to shape or grip the beads and which moves axially inwardly as the green tire expands radially (see U.S. Pat. Nos. 3,674,604; 3,833,445; 4,214,939; and 4,243,451).
Both the first- and second-stage tire building operations may be carried out on the same expansible drum as disclosed in many of the above-mentioned patents including U.S. Pat. Nos. 3,637,450; 3,684,621; 3,740,293; 3,795,564 and 3,873,397. On the other hand, it is common practice to build the carcass in stage one at one machine and then to transfer to another machine for application of the belts and the tread as disclosed for example in U.S. Pat. Nos. 4,131,500 and 4,151,035. A special expansible drum or other suitable means may be provided at the stage-II machine to expand the transferred stage-I carcass before the tread is applied as disclosed, for example, in U.S. Pat. Nos. 3,560,302; 3,674,604; 4,214,941 and 4,325,764.
Because of the problems involved with radial tires, special drums are employed for building the carcass in stage I. The old semi-automatic flat-band machines used for producing bias-ply truck tires, such as the NRM Models 49 and 59J described above, were generally considered unsuitable for that purpose.
Serious problems arise in the two-stage process when attempting to build tubeless radial tire carcasses on a cylindrical drum because of bead rotation problems when using wire beads of hexagonal-shaped or non-circular cross-section as are required in tubeless tires to maintain a proper seal at the wheel rim. If a tubeless truck tire carcass is built in cylindrical form in stage I and then expanded in stage II to toroidal shape, the bead rotation problem can be catastrophic because of the elongated cross section of the wire bead and the heavy construction in the bead region. For this reason such a process is unsuitable and not used for making standard heavy-duty tubeless truck tires. However, the process can be used for manufacturing tube-type truck tires having wire beads of generally circular cross-section (FIG. 13) or tubeless passenger car tires with a simple bead construction not seriously damaged by bead rotation in the stage II expansion.
The use of tubes in truck tires is highly undesirable particularly because of interference with tire cooling and the resulting decrease in service life. Heat build-up and resulting hot spots lead to early tire failure and rapid tire wear. Tubeless truck tires run cooler, are safer and have a longer service life. They are far superior to tube-type tires, but a high percentage of heavy-duty truck tires are still of the less expensive tube type. The special equipment and special methods heretofore required to build tubeless truck tires are slow and expensive and greatly limit the use of such tires. This problem has been recognized in the tire industry for several decades, and heretofore no practical solution was found. Prior to the present invention the industry had no simple and practical way to mass produce tubeless radial truck tires at low cost.
Some of the serious problems encountered in building tires are of small consequence when building smaller passenger car tires. For example, a practical and economical way to build a radial tire is the two-stage process commonly used for building passenger car tires wherein the carcass is built in cylindrical form on a cylindrical drum and then transferred to a stage II machine where it is mounted on a pair of axially movable bead-supporting rings, each being shaped like a train wheel and having a radially projecting annular rim flange for engaging and supporting the tire bead (see U.S. Pat. No. 3,923,572). The rim flange provides an air-tight seal at the bead so that the radial tire carcass can be expanded by internal air pressure in stage II from a cylindrical to a toroidal shape as the "train wheels" are moved axially towards each other.
The above process requires "buttoning" the cylindrical tire carcass over the "train wheels" so that both of the rim flanges are inside the tire beads. It is practical to do this with light-weight passenger car tires because the central half of the carcass is easily deformed. However, a heavy-duty truck tire carcass is too stiff to permit such a "buttoning" operation unless it is built in a semi-toroidal shape. Prior to the present invention in building tires the train wheels had to be reversed to locate the rim flanges on the outside and thereby eliminate the "buttoning" operation (see U.S. Pat. Nos. 3,342,663 and 4,214,941). This reversal of the flanges creates an air leakage problem during the stage II radial expansion of the cylindrical truck tire carcass. When leakage occurs the tire carcass is not supported properly during the expansion, the diameter of the expanded carcass cannot be controlled, and the quality of the tire is seriously denigrated. In spite of these deficiencies, the process continues to be used for manufacture of tube-type truck tires because of the higher rate of production and lower cost as compared to other more complicated methods.
During the last two decades, tire manufactures have attempted to improve the quality and reduce the cost of radial truck tires by use of complicated and expensive two-stage automatic tire building machines. Such machines are disclosed, for example, in U.S. Pat. No. 3,740,293 and in Rubber World, July 1979, pg 59. Bladder means are usually employed to carry out the ply turn up, turn down and stitching operations during building of the stage-I carcass. Unfortunately the machines have been too slow and inefficient to justify the large capital investment and have not provided tires of optimum quality. Another disadvantage of these expensive two-stage machines is the lack of versatility and the inability to produce truck tires in many different sizes. This is particularly true of tire building machines having drums with means to expand the tire to a near toroidal shape before the circumferential belts and the tread are applied. The construction of such drums is such as to interfere with or prevent the use of adjustable drums such as disclosed in U.S. Pat. Nos. 3,837,968 and 4,220,494.
The two-stage processes for building tubeless and tube-type radial tires encounter problems in quality control due to lack of precision in locating the bead rings and carcass material and the inability to control the bead-to-bead cord length during the tire building operation. The need for greater precision in the manufacture of radial truck tires has long been recognized.
For many years it has been common practice to build large tires on rigid high-crown drums having a diameter substantially greater than that of the wire bead rings as shown, for example, in U.S. Pat. No. 3,405,023 and on page 15 of Rubber Developments, Vol 30, No. 1, 1977. This procedure produces tires with defects near the bead region due to the wrinkles, folds and air pockets formed during the ply-down operation prior to setting of the beads and during the ply turn-up operation and does not provide the uniform high quality required in modern truck tires.